Feeding Your Baby

The new mom prepares a bottle with milk by shaking it with breast milk and formula.
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Can You Mix Breast Milk & Formula? Experts Weigh In

And how to ensure you’re mixing and storing safely

When it comes to feeding your baby, you have options. Some parents go the exclusively breastfeeding route, others do all formula, and some choose a mix of both. As long as your little one is fed, happy, and growing in a healthy way according to your pediatrician, any of those feeding methods are great. Serving up a mix of breast milk and formula is a popular option for parents who struggle with nursing or don’t produce enough breast milk to satisfy their baby, but it can also be a way to relieve some of the feeding pressure off mom.

At the end of the day, it doesn’t really matter why you’re choosing combination feeding, it just matters that you’re doing it the right way. Some parents might wonder about the logistics, like whether or not you can mix breast milk and formula in the same bottle, or if they should stay separate. If you want your baby to get the benefits of breast milk but find that you don’t have enough to fill a bottle, you might not know if you should hold off or top it off with some formula. You might also find that some pediatricians and experts have different answers. Here’s what you should know about mixing breast milk and formula before you try it.

What is combination feeding?

Combination feeding refers to feeding a baby both breast milk and formula. This can mean that you alternate between nursing and bottle feeding with formula or that you pump and bottle feed with both breast milk and formula or that you do a little bit of everything.

“Combination feeding is usually done when there are some problems with the baby, like if they’re not breastfeeding well enough alone to gain weight or mom has a low milk supply and needs to use formula to get an adequate amount to feed the baby,” explains Deedra Franke, registered nurse, International Board Certified Lactation Consultant at Mercy Medical Center. Of course, combination feeding can also be an option for mothers who are burnt out and need a reprieve from nursing or pumping.

Can you mix breast milk and formula in the same bottle?

When combination feeding, you might sometimes find that you don’t have enough breast milk to fill an entire bottle. In that instance, you have to make a decision: wait for the next pumping session to get enough breast milk for a full bottle, or add a little formula to get it where it needs to be. Is there a right or a wrong way? It depends who you ask.

“As a broad rule, I don’t recommend mixing them in the same bottle,” says board-certified pediatrician Maya Muñoz Mahmood, doctor of osteopathic medicine and Fellow of the American Academy of Pediatrics. “For a typically growing and thriving baby, it’s just not necessary.” She adds that while there are some specific and individual situations where a pediatrician might recommend mixing, it’s not something she would say to do regularly.

There aren’t any health risks involved in mixing breast milk and formula, but doing so could result in wasted breast milk. “If a baby does not finish the bottle they already started, the remainder needs to be thrown out within an hour, which is the rule for prepared formula or breast milk mixed with formula,” Mahmood says. “So, any remaining breast milk that was mixed in would be ‘wasted.’ For many parents who are pumping, every ounce of breast milk feels precious. So, they’d rather not risk having to toss any leftover breast milk.”

That said, if you think your baby will finish the bottle, then there’s nothing really wrong with mixing. “I don’t mind if parents mix breastmilk and formula,” Franke says. “In a NICU setting, breast milk is frequently fortified with formula prior to feeding.”

If you choose not to mix them together but want or need to give both, Mahmood recommends to offer the available breast milk first, then offer the formula in a separate bottle if the baby need more.

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How to mix breast milk and formula in the same bottle

If you want to mix breast milk and formula in the same bottle, Franke recommends doing so immediately before the feeding session. Don’t prepare it a few hours before and then let it sit. If you’re using powdered formula, mix it together on its own, then add it to the breast milk when the baby is ready to eat. If you’re using breast milk and ready-to-feed formula, you can mix them up right before baby eats.

How to mix and store breast milk and formula safely

In terms of safety, you really want to be aware of how long both the breast milk and the formula are sitting out. The Centers for Disease Control and Prevention (CDC) recommends that prepared formula at room temperature should be used within two hours of preparation or within one hour from when feeding begins. Breast milk can sit at room temperature for four hours. To be as safe as possible, mix the bottle right before feeding. Once breast milk is mixed with formula, that bottle only has two hours to be used.

Prepared formula can be stored in the refrigerator for up to 24 hours, and open containers of ready-to-made formula can be stored in the refrigerator for up to 48 hours. Freshly expressed breast milk can be in the fridge for a longer period of time — up to four days — while thawed frozen breast milk can be in the fridge for one hour. If you’re going to mix breast milk and formula, err on the side of caution and do not leave it in the refrigerator for more than 24 hours.

Warming up a mix of breast milk and formula can be confusing as well. Franke recommends warming the stored breast milk and formula individually before mixing them together to ensure that neither one gets too hot. You can do this by running the bottle under warm tap water or using a bottle warmer at the correct setting. “Baby bottles should never be microwaved due to a risk of scalding injuries,” Madmood warns. “When a bottle is microwaved, it heats unevenly, so there may be a ‘hot spot’ that can burn a baby's mouth.”

Where are no health risks to mixing breast milk and formula, it’s not an ideal option if there’s a chance your baby might not finish their bottle, since you don’t want to risk losing out on any breast milk. It can also make storage a little confusing if you mix them ahead of time. That said, if it makes your life easier and doesn’t seem to be bothering your baby, there’s nothing wrong with doing it — just be sure to keep CDC guidelines in mind for room temperature and refrigerator storage to ensure the bottle doesn’t go bad.


Deedra Franke, Registered Nurse, International Board Certified Lactation Consultant at Mercy Medical Center

Maya Muñoz Mahmood, board-certified pediatrician, doctor of osteopathic medicine, and Fellow of the American Academy of Pediatrics